local ENT = Entity(1027)
local size = 1024

local max_size = 5

local count = 10000
local sand = {}

for i=1, count do
    local siz = math.Rand(1,max_size)
    sand[i] =
    {
        pos = {x = size*0.5, y = size*0.5},
        vel = {x = 0, y = 0},
        siz = siz,
        drag = (0.99 - (siz / (max_size * 80))),
    }
end

local drawpixel

do
    local col = {r=0, g=0, b=0, a=0}
    local cache = {}
    local vel

    local SetDrawColor = surface.SetDrawColor
    local DrawLine = surface.DrawLine
    local floor = math.floor

    drawpixel = function(data)
        vel = floor(data.vel.x + data.vel.y)

        -- cache the color
        col = cache[vel] or HSVToColor(vel, vel, 1)
        cache[vel] = col

        SetDrawColor(
            col.r, col.g, col.b,
            255
        )

        DrawLine(
            data.pos.x,
            data.pos.y,

            data.pos.x + data.vel.x * -0.2,
            data.pos.y + data.vel.y * -0.2
        )
    end
end

local new
local function localtoworld(vec, ang)
    new = WorldToLocal(vec, Angle(0), Vector(0), ang)
    new:Rotate(Angle(180,0,0))
    return new.x, new.y
end

local hash_map = {x={},y={}}

local function calc_self_collision(part)
    local x = math.Round(part.pos.x, 2)
    local y = math.Round(part.pos.y, 2)

    if hash_map.x[part.last_hash_x] then
        hash_map.x[part.last_hash_x] = false
        hash_map.y[part.last_hash_y] = false
    elseif hash_map.x[x] and hash_map.y[y] then
        part.vel.x = part.vel.x * -0.5
        part.vel.y = part.vel.y * -0.5
    end

    hash_map.x[x] = true
    hash_map.y[y] = true

    part.last_hash_x = x
    part.last_hash_y = y
end

local WorldSound = WorldSound
local clamp = math.Clamp
local sqrt = math.sqrt
local random = math.random

local function play_sound(part)
    local len = clamp(sqrt(part.vel.x^2 + part.vel.y^2) + (random() * 0.1), 1, 100)
    if len > 30 then
        WorldSound("physics/flesh/flesh_bloody_impact_hard1.wav", ENT:GetPos(), len*0.5, 100 + math.random(1, 10) + (((-part.siz / max_size) + 1) * 200) )
    end
end

local function calc_collision(part)
    if part.pos.x - part.siz < 0 then
        play_sound(part)
        part.pos.x = part.siz
        part.vel.x = part.vel.x * -part.drag
    end

    if part.pos.x + part.siz > size then
        play_sound(part)
        part.pos.x = size - part.siz
        part.vel.x = part.vel.x * -part.drag
    end

    if part.pos.y - part.siz < 0 then
        play_sound(part)
        part.pos.y = part.siz
        part.vel.y = part.vel.y * -part.drag
    end

    if part.pos.y + part.siz > size then
        play_sound(part)
        part.pos.y = size - part.siz
        part.vel.y = part.vel.y * -part.drag
    end
end

local delta
local ext_vel_x, ext_vel_y
local rand = math.Rand

local function DrawScreen()
    surface.SetDrawColor(50,50,50,255)
    surface.DrawRect(0,0,size,size)

    -- external velocity
    ext_vel_x, ext_vel_y = localtoworld(ENT:GetVelocity(), ENT:GetAngles())
    ext_vel_x = ext_vel_x * 0.3
    ext_vel_y = ext_vel_y * 0.3

    -- gravity
    local grav = Vector(0,0,0)--ENT:WorldToLocalAngles(physenv.GetGravity():Angle()):Up() * 5
    ext_vel_x = ext_vel_x + grav.y
    ext_vel_y = ext_vel_y + grav.x

    delta = FrameTime() * 5

    for i, part in pairs(sand) do
        -- random velocity for some variation
        part.vel.x = part.vel.x + ext_vel_x + rand(-2,2)
        part.vel.y = part.vel.y + ext_vel_y + rand(-2,2)

        -- velocity
        part.pos.x = part.pos.x + (part.vel.x * delta)
        part.pos.y = part.pos.y + (part.vel.y * delta)

        -- friction
        part.vel.x = part.vel.x * part.drag
        part.vel.y = part.vel.y * part.drag

        -- collision
        calc_collision(part)

        -- collision with other particles (buggy)
        --calc_self_collision()

        drawpixel(part)
    end
end

function ENT:RenderOverride()
    self:DrawModel()
    cam.Start3D2D(self:GetPos() + self:GetRight() * -25 + self:GetForward() * -25 + self:GetUp() * 2, self:GetAngles(), 0.048)
        DrawScreen()
    cam.End3D2D()
end

ENT:SetRenderMode(1)